What does the diagonal length mean on a TV or monitor?

The diagonal is the straight-line distance from one corner of the panel to the opposite corner. Manufacturers and retailers quote screens this way — a "55-inch TV" has a 55-inch diagonal — because it summarises both width and height in a single number. To know how wide and tall the actual rectangle is, you need the aspect ratio as well.

What is an aspect ratio, and why does it matter?

An aspect ratio expresses the screen's width relative to its height as a pair of whole numbers, like 16:9 (sixteen units wide for every nine tall). Two screens with the same diagonal but different aspect ratios have very different widths and heights — a 27-inch 21:9 ultrawide is much wider but much shorter than a 27-inch 16:9 monitor. If you're fitting a screen into a cabinet, the width and height matter, not just the diagonal.

How is 21:9 different from 16:9?

21:9 (often labelled "ultrawide") is wider and shorter than 16:9 for the same diagonal. The trade-off: more horizontal pixels for cinematic films and side-by-side workflows, less vertical room for documents and web pages. Modern ultrawide monitors are typically 21:9; a few "super-ultrawide" panels go as wide as 32:9.

Does this calculator work for a square 1:1 display?

Yes. With a 1:1 aspect ratio the rectangle becomes a square, so width and height are equal — both come out to the diagonal divided by √2. Square panels are uncommon on consumer TVs but show up in industrial signage, control-room monitors, and a few productivity displays.

How do I find the diagonal on my spec sheet?

Manufacturer pages usually list the diagonal under "screen size" or "display size", quoted in inches even in metric markets. If the spec sheet only gives width and height, the diagonal is √(width² + height²) — but for buying decisions it's usually quicker to just enter the diagonal you already see on the product page.

GEOGeometry

Screen Size Calculator

Turn a manufacturer's diagonal spec — 27-inch monitor, 55-inch TV — into the actual width and height of the panel. Pick the aspect ratio (16:9, 21:9, 16:10, 4:3, 1:1, or a custom pair), enter the diagonal, and you'll know whether the screen fits the desk, the cabinet, or the wall niche before you buy.

Aspect ratio

Pick a standard ratio, or choose Custom to enter your own.

Diagonal length

Custom width units(optional)

Custom height units(optional)

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How It Works

Formula

W = \dfrac{d \cdot w}{\sqrt{w^2 + h^2}}

H = \dfrac{d \cdot h}{\sqrt{w^2 + h^2}}

A = W \cdot H

Where

$d$: Diagonal length quoted by the manufacturer
$w$: Aspect-ratio width units (e.g. 16 in 16:9)
$h$: Aspect-ratio height units (e.g. 9 in 16:9)
$W$: Actual screen width
$H$: Actual screen height
$A$: Screen area (W × H)

A diagonal and an aspect ratio describe a right triangle whose two legs are the screen's width and height — the diagonal is the hypotenuse. The calculator divides the diagonal by √(w² + h²) to get the unit length, then multiplies by w and h to recover the actual width and height. Multiplying the two gives the screen area.

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